1. Field of the Invention
The present invention relates to an electromagnetic interference suppressing device and a circuit for suppressing electromagnetic noise arising in the periphery of a semiconductor circuit mounted on a multilayer printed circuit board.
This application is based on Japanese Patent Application No. 11-300396, the contents of which are incorporated herein by reference.
2. Description of the Related Art
In recent years, as the operating speeds of semiconductor products such as transistors, ICs, and LSIs have increased, there have been the serious problems of EMI (Electro-Magnetic Interference) which causes malfunctions in electronic devices containing the semiconductor devices or in other electronic devices.
For example, in recent personal computers, the internal clock speed of the CPU (Central Processing Unit) has increased to 650 MHz, and is expected to further increase to the order of GHz. Signal lines or power supply lines of LSIs, driven at high frequencies, contain high frequency components operating above several GHz, which regularly causes electromagnetic noise at high frequencies. Therefore, in multilayer printed circuit boards on which a number of semiconductor devices are mounted without an appropriate means for reducing the noise, the connections on the board act as antennae, and electromagnetic noise is emitted as radio waves, which may cause malfunctions in electronic devices or in other electronic devices.
EMI is mainly caused by electromagnetic radiation due to an electric current (roundabout current) which is called common mode, caused by parasitic mutual inductance or parasitic capacitance, or by a high frequency current flowing in the power supply lines. The cause of EMI is, however, complex, and there is no effective countermeasure which is applied in proximity of the sources.
A technique has been proposed for providing an electromagnetic noise absorption layer, for absorbing electromagnetic noise, on the upper and lower sides of the printed circuit board, or for providing the electromagnetic noise absorption layer as an internal layer. The technique cannot control the occurrence of the electromagnetic noise, and its effectiveness is limited. In general, a metal case, as a measure for preventing electromagnetic noise, is used to shield the entire electronic device.
In a general multilayer printed circuit board, the power source layer, the ground layer, and signal layers are layered with an intervening insulating material. In a multilayer printed circuit board shown in FIG. 9, an IC/LSI803, which is a source of a high frequency source current, is connected between a power supply line of a source layer 801 of the multilayer printed circuit board 805, and a ground line of a ground layer 802. A decoupling capacitor 804 is located in proximity of the IC/LSI803, and is connected in parallel between the source layer and the ground layer.
The decoupling capacitor 804 allows the high frequency source current, which flows through the source layer 801 depending on the switching operation of the IC/LSI803, to bypass the IC/LSI803. Further, a variation in voltage at a source terminal 803A of the IC/LSI803 caused by the switching operation of the IC/LSI803 can be suppressed.
In the conventional multilayer printed circuit board 805, the source layer 801, which serves as the power supply line, is a planar source layer completely covering the substrate area and comprising a conductive film. The whole-area plane source layer maximizes the area through which the electric current flows, thus reducing the resistance of the power supply line, and decreases linear variations in the source voltage.
An EMI suppressing technique for a multilayer printed circuit board has been proposed in which the source layer is formed as lines to control high frequency currents (Japanese Patent Application, First Publication No. Hei 9-139573). FIG. 10 is a plan view showing a principal part of a printed circuit board using this technique, and showing the layout of the source layer on the multilayer printed circuit board 901. On the multilayer printed circuit board 901 shown in FIG. 10, the source layer (hatched area) comprises a number of branch source connections 905 with comb or zigzag shapes which branch from the main line pattern 902 which is a main conductive portion.
Circuit elements (semiconductor integrated circuits) 903 are located at the ends of the branch source lines 905. The power is supplied to the circuit elements 903 through the main line pattern 902 and the branch source connections 905. Decoupling capacitors 904 are provided for the respective circuit elements 903 at the power supply points and at the connection points between the main line pattern 902 and the branch source connections 905.
The conventional example is characterized in that, because the branch source connections 905 act as inductance elements, a comparatively high inductance can be obtained in the power supply circuit for the respective circuit elements 903. Therefore, this technique reduces the source current at high frequencies which is caused by the switching operation of one of the circuit elements 903 and which flows through the decoupling capacitors of the other circuit elements 903, as compared with the conventional printed circuit board. Namely, the source layer of the line patterns acts as a circuit for producing impedance, and enhances the filtering effects of the decoupling capacitors.
The conventional example of the multilayer printed circuit board 805 with the whole-area plane source layer 801 causes the problem that a designer cannot adjust the high frequency source current flowing through the decoupling capacitors 804 to the source layer 801 at the time of switching the IC/LSI803. Because the impedance of the whole-area plane source layer 801 is low, the high frequency source current from the IC/LSI803 flows through not only the decoupling capacitor 804 in proximity of the IC/LSI803 but also the other decoupling capacitors 804 in proximity of the other IC/LSIC803. The distribution of the high frequency source current over the entire multilayer printed circuit board 805 is complicated, and is difficult to analyze. Therefore, the capacities of the decoupling capacitors 804 prepared for the respective IC/LSI803 cannot be specified.
Further, because the source layer 801 forms the whole-area plane, the high frequency source current flows through a complicated path in the source layer 801, and may form a large loop which causes electromagnetic noise.
FIG. 11 is a circuit diagram showing the periphery of a plurality of circuit elements connected to the decoupling capacitors. In this example, an IC/LSI 101a whose high frequency source current is high, an IC/LSI 101b whose high frequency source current is medium, and an IC/LSI 101c whose high frequency source current is low are connected to the power supply line and to the ground in a parallel manner. Decoupling capacitors are provided, depending on the amounts of the high frequency source currents of the IC/LSI 101a to 101c: a decoupling capacitor 102a whose capacity is high (impedance is low) is provided in proximity of the IC/LSI 101a, a decoupling capacitor 102b whose capacity is medium (impedance is medium) is provided in proximity of the IC/LSI 101b, and a decoupling capacitor 102c whose capacity is low (impedance is high) is provided in proximity of the IC/LSI 101c. 
As shown in the example of the multilayer printed circuit board 805 with the whole-area plane source layer 801, if the inductances 103a, 103b, and 103c are not provided, the following problem may arise.
Because the impedance of the decoupling capacitor 102c located near the IC/LSI 101c is high, a part of the high frequency source current from the IC/LSI 101c is not released to the ground through the decoupling capacitor 102c, and may flow through the IC/LSI 101a or 101b, increasing the area of the loop of the electric current, and thus increasing the noise due to the electromagnetic radiation.
When the high frequency source current from the IC/LSI 101c is not sufficiently released through the decoupling capacitor 102c, the high frequency source current flows through the other paths, and the source voltage may vary like an alternating current, which may adversely affect the operation of the IC/LSIs.
Further, the conventional technique accommodates the electronic device in the metal case for suppressing the electromagnetic noise to be radiated to the outside of the device. The metal case, however, requires an opening through which a console for the electronic device is provided. Therefore, the conventional technique cannot completely prevent the leakage of electromagnetic noise.
In the multilayer printed circuit board with the source layer of the line patterns, the source layer is equivalent to the inductances 103a, 103b, and 103c as shown in FIG. 11. The decoupling capacitors 102a to 102c near the IC/LSI 101a to 101c allow the high frequency current to bypass the other IC/LSI 803, thereby decreasing the electric current loop.
This technique, however, makes the line patterns of the source layer complicated. To obtain the sufficient inductances, a large area is needed, and the packaging density therefore decreases.
Moreover, the decoupling circuits must be designed for a number of the respective high frequency switching circuit elements and their source-ground terminals, and the number of design steps may be significantly increased. The design of the decoupling circuits requires the data of the high frequency source current of the high frequency switching circuit element (IC, or LSI), and the characteristic impedance and electric current characteristics of the power supply system, which are not in general disclosed by semiconductor manufacturers. Since the values of the high frequency source currents must be estimated, based on the available characteristic data, the decoupling circuits cannot be accurately designed.
Further, the frequency characteristics of the ceramic capacitors used in the decoupling circuit are in general unsatisfactory, and the ceramic capacitors may function as inductors so that the expected decoupling effects cannot be achieved. For example, the resonance frequency of a ceramic capacitor of 0.1 xcexcF is around 10 MHz, and the ceramic capacitor may function as an inductor when above that frequency. The reason for this is that inductance components such as the electrode pattern of the capacitor and lead wire are present in series with the capacitance.
The parasitic inductance of a general capacitor depends on its dielectric material, electrode pattern structure, and capacitance value, and is approximately 2nH in the case of a chip capacitor, and is approximately 7nH in the case of a capacitor with two terminals and lead wires.
It is therefore an object of the present invention to provide an electromagnetic interference suppressing device and a circuit which can effectively suppress electromagnetic noise radiated around a semiconductor integrated circuit.
It is another object of the present invention to provide an electromagnetic interference suppressing device and a circuit which can suppress variations in source voltage caused by high frequency source currents as a semiconductor integrated circuit operates, to stabilize the operation of the semiconductor integrated circuit.
It is another object of the present invention to provide an electromagnetic interference suppressing device and a circuit which can significantly reduce the number of steps for designing a power supply decoupling circuit, eliminating a complex design process.
It is another object of the present invention to provide an electromagnetic interference suppressing device and a circuit which can significantly reduce the number of steps for designing a power supply decoupling circuit, eliminating a complex design process. The electromagnetic interference suppressing device of the present invention comprises a plurality of connection layers and ground layers formed of a conductive material. The connection layers and the ground layers are alternately layered. Insulating layers formed of an insulating material intervene between the neighboring connection layers and ground layers. The odd connection layers counting from the bottom and the connection layers just above those layers are electrically connected at a first end. The even connection layers counting from the bottom and the connection layers just above those layers are electrically connected at a second end opposite to the first end. The bottommost connection layer is connected to a first signal terminal. The uppermost connection layer is connected to a second signal terminal. The ground layer is connected to a ground terminal.
The electromagnetic interference suppressing circuit of the present invention comprises a plurality of connection layers and ground layers formed of a conductive material. The connection layers and the ground layers are alternately layered. Insulating layers formed of an insulating material intervene between the neighboring connection layers and ground layers. The odd connection layers counting from the bottom and the connection layers just above those layers are electrically connected at a first end. The even connection layers counting from the bottom and the connection layers just above those layers are electrically connected at a second end opposite to the first end. The lowest connection layer is connected to a first signal terminal. The uppermost connection layer is connected to a second signal terminal. The ground layer comprises an electromagnetic interference suppressing device connected to the ground terminal, and a capacitor connected between the first or second signal terminal and the ground terminal. The characteristic impedance of the capacitor are approximately the same as the characteristic impedance of the electromagnetic interference suppressing device between the first or second signal terminal connected to the capacitor and the ground terminal.
The circuit equivalent to the electromagnetic interference suppressing device of the present invention is a structure in which a plurality of coils are connected in series, and in which capacitors, provided by the connection layers and the ground layers, are connected between the connection points of neighboring coils and the ground terminal. Therefore, a lower characteristic impedance of 0.05 to 0.1xcexa9 can be achieved.
In the electromagnetic interference suppressing circuit of the present invention, the electromagnetic interference suppressing device is connected to a capacitor. When the capacitor is a ceramic capacitor, the electromagnetic interference suppressing device is terminated with the same impedance as the characteristic impedance because the equivalent series resistance of the ceramic capacitor is approximately 0.05 to 0.1xcexa9, thereby reducing reflections in the transmission line.
When the first or second terminal of the electromagnetic interference suppressing device, which is not connected to the capacitor, is connected to the source terminal of a semiconductor device, the source terminal is connected to the ground with a extremely low impedance at high frequencies, and the high frequency source current arising in the semiconductor device immediately bypasses the source terminal through a short loop.
As a result, only a small amount of the high frequency source current produced by the semiconductor device flows through the source conductor, thereby significantly reducing the electromagnetic noise radiated from the source conductor acting as an antenna.
The electromagnetic interference suppressing circuit allows the high frequency source current arising from other semiconductor devices to bypass the semiconductor device connected to the electromagnetic interference suppressing circuit, thereby improving the noise tolerance.
Because only a small amount of the high frequency source current produced by the semiconductor device flows through the source conductor, the alternating variations in the source voltage can be significantly suppressed, thereby stabilizing the operation of the semiconductor device.
Further, the electromagnetic interference suppressing circuit achieves the same function as that of the conventional decoupling capacitor, and can therefore eliminate the conventional decoupling capacitor connected to the source terminal of the semiconductor integrated circuit. The electromagnetic interference suppressing circuit eliminates the design steps of using a decoupling capacitor in consideration of the characteristics of the respective semiconductor circuits, thereby significantly reducing the number of the design steps.
The electromagnetic interference suppressing device basically has the structure in which the conductive layers are layered with the intervening insulating layers, thereby simplifying the structure, lowering the costs, and reducing the size of the device.
Further, because it is not necessary to increase the inductance by the zigzag source connection, the semiconductor devices can be mounted with a high density on the multilayer printed circuit board.